The ion implantation delamination method is a method of producing a SOI wafer by bonding of a wafer into which hydrogen ions or rare gas ions and a wafer and delamination. However, there is a case where a SOI layer is not transferred on an edge portion of the SOI wafer after delamination, and accordingly a terrace portion where a surface of a base wafer (support substrate) is exposed is generated. This is mainly due to that the bonding strength between the bonded wafers is weak in the edge portion of the wafers, and therefore, a SOI layer is difficult to be transferred on a base wafer side. When the SOI terrace portion was observed by a optical microscope, it was found that SOI islands which are pieces of isolated island-shaped SOI layer were generated in the edge portion of the SOI layer. It is expected that such SOI islands come off from the wafer by etching to eliminate a buried oxide film (may be called a BOX oxide film) during a cleaning by use of an aqueous solution containing HF (hydrofluoric acid) in a device fabrication process, and accordingly, they cause device failure because they adhere to a device fabrication region of the wafer again as silicon particles.
FIG. 1 show cross sectional views of an edge portion of the SOI wafer produced by an ion implantation delamination method.
FIG. 1(a) is a SOI wafer 10, and FIG. 1(b) shows the detail of its edge portion. FIG. 1(b) schematically shows the state that the SOI wafer 10 comprises a SOI layer 25, a buried oxide film 26 and a base wafer 27, and a terrace portion 43 where a surface of the base wafer is exposed and SOI islands 42 which are pieces of isolated island-shaped SOI layer are generated in the edge portion.
On the other hand, as SOI wafers produced by the ion implantation delamination method have been observed by an optical surface inspection apparatus, it has been found that defects detected as LPD exist. The LPD (Light Point Defect) is a generic term of the defect, which appears as a spot when a surface of the wafer is observed under a condenser lamp. Although this defect has not been identified, it is considered that a device yield is affected because nearly all the defects are shallow pits and become holes which pass through a SOI layer when a SOI layer is oxidized to be thin.